In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. To the demand for a resist material with a higher resolution and sensitivity, chemical amplification positive working resist materials which are catalyzed by acids generated upon light exposure are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography. Also, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) brought about a significant innovation. Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, passed through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated.
For ArF laser (193 nm), it is expected to enable miniaturization of the design rule to 0.13 μm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
With respect to F2 laser (157 nm) which is expected to enable further miniaturization to 0.10 μm or less, more difficulty arises in insuring transparency because it was found that acrylic resins which are used as the base resin for ArF are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. It was also found that poly(vinyl phenol) which is used as the base resin for KrF has a window for absorption in proximity to 160 nm, so the transmittance is somewhat improved, but far below the practical level.
For improving the transmittance in proximity to 157 nm, reducing the number of carbonyl groups and/or carbon-to-carbon double bonds is contemplated to be one effective way. It was also found that introducing fluorine atoms into base polymers makes a great contribution to improved transmittance. In fact, poly(vinyl phenol) having fluorine introduced in its aromatic rings offers a transmittance nearly on a practically acceptable level. However, this base polymer was found to turn to be negative upon exposure to high-energy radiation as from an F2 laser, interfering with its use as a practical resist. In contrast, those polymers obtained by introducing fluorine into acrylic resins or polymers containing in their backbone an alicyclic compound originating from a norbornene derivative were found to be suppressed in absorption and overcome the negative turning problem.
It was recently reported that copolymers of t-butyl α-trifluoromethylacrylate with 5-(2-hydroxy-2,2-bistrifluoromethyl)ethyl-2-norbornene and copolymers of t-butyl α-trifluoromethylacrylate with 3-(hydroxybistrifluoromethyl)methylstyrene are suited to form resists having high dry etching resistance and high transparency (SPIE 2001, Vol. 4345-31; Polymer design for 157-nm chemically amplified resists). However, these copolymers have an absorbance of the order of 2 to 3, and the resist pattern reported in the article merely has a thickness of about 1,000 Å. A further improvement in transmittance is necessary. It is generally believed that an absorbance of no more than 2 is necessary in order to produce rectangular patterns at a thickness of 2,000 Å or greater. Heretofore, the resist material capable of satisfying all the requirements including dry etching resistance, alkali solubility, adhesion and transparency is not available in the art.
In order to process the underlying substrate through a very thin film resist pattern having a thickness in the range of 1,000 to 1,500 Å, the resist material must have very high resistance to etching.
The polymers of α-trifluoromethylacrylic acid derivatives, whose polymerization method is described in A.C.S. Macromolecules, 1982, 15, 915-920, have been under study as the resist of the backbone decomposition type adapted for electron beam exposure.
JP-A 9-43848 discloses α-trifluoromethylacrylates having a cyclic structure, which serve as non-chemically amplified resists of the backbone decomposition type.